Criminals may have stolen more than half a million credit card numbers from merchant servers hosted by Networks Solutions, the Internet hosting company warned Friday.

In a letter sent to merchants who use its Ecommerce Hosting services, the company said that someone illegally installed software on company servers used handle credit card transactions initiated by 573,928 people between March 12 and June 8, 2009.

The code "may have been used to transfer data on certain transactions for approximately 4,343 of our more than 10,000 merchant Websites outside the company," Network Solutions said in the letter, signed by company chairman and CEO Roy Dunbar and sent to merchants on Friday.